Sudden cardiac death (SCD) is the major cause of cardiac mortality and affects over 400,000 individuals per year in the United States. SCD often occurs in patients with coronary artery disease, cardiomyopathy, and ion channelopathies. SCD may be caused by a ventricular tachyarrhythmia that degenerates into ventricular fibrillation. Less commonly, bradyarrhythmias are associated as the initiating event of sudden cardiac death.
Many patients at risk of sudden cardiac death have pacemakers, implantable cardioverter defibrillators (ICDs) or other medical devices implanted therein. These devices may continuously obtain cardiac signals that are digitized and presented as electrocardiograms (ECG). ECGs are composed of various waves and segments that represent the heart depolarizing and repolarizing. The ST segment in an ECG represents the portion of the cardiac signal between ventricular depolarization and ventricular repolarization. While P-waves, R-waves, and T-waves in the ECG may generally be considered features of a surface ECG, for convenience and generality, herein the terms R-wave, T-wave, and P-wave are also used to refer to the corresponding internal cardiac signal, such as an intra-cardiac electrogram (IEGM) signal. Additionally, modern implant devices also may provide other types of cardiac related signals such as blood pressure and impedance in a continuous or periodical nature.
ECG, IEGM, pressure measurements, and impedance measurements may be useful for monitoring unstable myocardial behavior within the heart, which may lead to the onset of ventricular tachyarrhythmias in the future. Yet, in spite of the known risk stratification methods, accurate prediction of the onset of sudden cardiac death remains a relatively difficult and imperfect task. Additionally, the known implantable devices hardware may limit the usage of comprehensive algorithms that exceed device capability. Thus, a method and system are needed for implantable devices to track myocardial instability, predict the onset of sudden cardiac death, and/or deliver preventive therapy.